In spite of the
basically environment-oriented objective of wastewater
disposal, various problem factors may arise which may be
impossible or difficult to overcome:

a)
technically/economically unavoidable emissions (residual
emissions), from the wastewater disposal installations which have
an overall impact on air, soil and water, on people and on
ecosystems

b) unforeseen increase
in volume of sewage from dwellings (due to changes in lifestyle)

c) unforeseen increase
in volume of sewage from commercial and industrial establishments
(due to production increases, fluctuations, seasonal operation)

d) eutrophication
phenomena in the receiving body of water into which the treated
wastewater is discharged, during long periods of low rainfall

e) adverse consequences
of using sewage sludge or composted refuse/sewage sludge in
agriculture for the purposes of recycling.

From the start,
reasonable allowance should be made for the above-mentioned problem
factors in all project management activities, in order to minimise
from the outset any conceivable effects, using suitable measures
of an organisational, structural and operational nature, and
in certain circumstances also with recourse to emergency
measures. Furthermore, wastewater disposal measures are to be
planned taking into account local conditions so that they
satisfy the generally accepted rules of wastewater technology or
the state of the art (after careful consideration where
necessary).

Considering each
relevant sub-sector of wastewater disposal in turn, the potential
typical environmental impacts are as follows (7), (8).

2.2 Typical
environmental impact

In any wastewater
disposal project, it is vital to decide whether to adopt:

- a decentralised
local sewerage system (individual sewerage system at each
source with wastewater collection pits, cesspits or small
sewage works, latrines, etc.) or
- a central local sewerage system (collective sewerage system
comprising a sewer network with all necessary installations,
to collect, divert and deliver the wastewater produced at the
individual sources to one or more (central) sewage treatment
works(s)

Different
environmental effects occur here, the most important of which
are set out below.

2.2.1 Impact of
wastewater collection and removal

2.2.1.1 Decentralised
sewerage system

Decentralised wastewater
disposal may have the following adverse effects on the
environment:

For the user, individual
sewerage systems mean higher expenditure on maintenance
and upkeep compared with systems connected to a central
sewerage system. If not properly constructed this can lead
to problems such as

- poor quality of
the effluent from the small sewage works (if the sludge
settling chambers are not regularly and properly emptied) and
hence the contamination of the receiving body of water,
- frequent running of emergency overflows from pump shafts if
the pumps are not properly maintained and hence contamination
of areas which should only receive effluent in definite cases
of emergency,
- contamination of the subsoil and particularly of
groundwater in the case of dry closets (aerated/unaerated
latrines), cesspools, percolation systems (particularly
soakaways after aerobic or anaerobic biological wastewater
treatment) leaking cesspits etc. in particular where the
hydrogeological conditions are unsuitable,
- health hazards due to the operation of individual sewerage
systems (e.g. danger of infection through direct contact when
emptying pit contents; through insects and rat infestation),
- health hazards upon final disposal (discharge) of sludge
from small sewage works or pit contents from collection
cesspits, where not properly executed,
- aesthetic and odour nuisance,
- no possibility of central removal and treatment of
commercial/industrial wastewater together with domestic
sewage.

Beneficial effects of
decentralised wastewater disposal on the environment may be as
follows:

- The natural water
cycle is scarcely interrupted or disturbed by the proper
collection and removal of rainwater (this considerably
reduces the amount of percolation from rainwater).
- There may be a greater incentive to reduce water
consumption (increased consumption will lead to substantially
higher costs of removal of pit contents).
- (Inefficient) receiving bodies of water are not subject to
sporadic or sudden contamination at the rainwater overflows
and outfalls or at the outfall works of sewage treatment
plants.
- Eutrophication or desertification phenomena in receiving
bodies of water are largely eliminated.

2.2.1.2 Central sewerage
system

The beneficial
effects of a decentralised local sewerage system
mentioned in 2.2.1.1. are absent in the case of a central
local sewerage system (central wastewater disposal). Indeed,
their absence is a positive disadvantage. In addition, a central
sewerage system has the following adverse effects on
the environment:

- With inadequately
designed or manufactured pipe couplings, serious leakage may
occur (penetration of groundwater; hydraulic overload of
pipes, pumping station and sewage works; leakage of
wastewater; contamination of subsoil or groundwater).
- With large wastewater pumping stations, noise and odour
nuisance may occur if

·they are too close to neighbouring
buildings or

·sound insulation, aeration, ventilation
and deodorization are lacking or inadequate.

Major positive
effects of a central sewerage system are in particular the
following (9):

- protection of the
population from health hazards caused by infectious germs
transmitted directly or indirectly by water, e.g. in the
event of contamination of groundwater used for individual
drinking water supplies or by direct contact with the
wastewater.
- protection of the population from aesthetic nuisance caused
by substances in wastewater which readily putrefy.
- protection of the population from flooding of cellars and
storage rooms during storms.
- safeguarding of motor vehicle, bicycle and pedestrian
traffic, even in case of heavy rainfall.
- possibility of removal and treatment of commercial or
industrial wastewater together with domestic sewage.
- protection of utilisable groundwater reserves from
contamination by substances contained in (domestic) sewage,
especially nitrogen compounds.

2.2.1.3 Special
wastewater disposal processes

In certain wastewater
disposal areas, a combination of these two disposalsystems may be appropriate. In some cases it may also be
worthwhile in terms of ecology and water management to have a sewerage
system in which only the sewage is centrally
removed, but not the rainwater.

Moreover with the dual
system, overground or underground removal of rainwater may also
be a good idea in terms of ecology and water management if the
sewage and rain water are consistently separated, taking care
to ensure that the rainwater remains as "clean" as
possible. In other words: The rainwater, which is relatively
clean from the outset, should not be deliberately mixed with a
medium (in this case sewage) whereby it likewise becomes a dirty
medium. With careful and proper operation of such a dual
system, the pollutant load in the receiving bodies
of water can be substantially reduced, for the
following reasons in particular:

- There is no need
for rainwater or mixed water outfalls, therefore there are no
mixed water effluents, which can otherwise cause serious
pollution of receiving bodies of water, particularly after
long dry spells.
- Only sewage passes to the central treatment works,
so that the wastewater flow at the intake is considerably
reduced and homogenized, thus considerably improving the
efficiency and safety of the works.

2.2.2 Impact of
wastewater treatment

2.2.2.1 Introductory
notes

The qualitative and
quantitative criteria for proper wastewater treatment -
and so for its environmental impact - are derived
primarily from emission and immission standards, which in
turn are derived from the relevant water management conditions
and from the legislation, regulations etc. in force. In many
countries the latter rarely exist or where they do exist are
inadequate. Direct application of, say, German, EC or
American laws and regulations rarely provides an appropriate
solution. Rather, it is necessary to develop measures
suited to the prevailing general constraints and to implement
them with the involvement of the local population.

2.2.2.2 Emissions from
(central) sewage works

The substances in
wastewater which pollute the water and sewage sludge in a
public (municipal) wastewater disposal plant require a variety
of processes and facilities to eliminate or reduce
them. When planning a sewage works these are combined in a
certain way or arranged in series (treatment stages). Table 1
summarises the technically feasible processes for treatment of
municipal wastewater on the basis of the present level of
development - with their treatment capacities expressed as
the degree of efficiency (5).

The procedures in
question are listed in the sequence in which they normally
occur in the treatment stages of a sewage works, in
order to achieve optimum results. Most importantly, the table
shows the anticipated impact on the receiving body of water,
i.e. pollutant emissions as a percentage of the concentrations in
the incoming wastewater.

For the efficiency of
anaerobic wastewater treatment processes (highly suitable in
countries with hot climates) please refer to (10).

Sewage works affect the
environment not only in terms of water-related emissions, but
also in terms of

- noise
- odours and
- air pollution (aerosols).

As a rule, however, it
can be assumed that these types of emissions are less important
than the water-related emissions of a sewage works (wastewater
discharge).

2.2.2.3 Emissions of
small sewage works

In the case of decentralised
wastewater disposal systems individual or small sewage
works may be involved (see 2.2.1.1). The disposal of the
treated wastewater can either take place via discharge
into a body of surface water or via discharge into the subsoil
(seepage, percolation).

As regards permissible
emissions of small sewage works and their environmental
impact one must basically make a distinction between two
types of works:

a) works without
wastewater aeration, also called septic tanks (with exclusively
mechanical or partly biological (anaerobic) treatment) and

b) works with wastewater
aeration and mechanical/biological treatment. With works of type
a), biological efficiency of 20-25% and in exceptional
cases as much as 50% is obtained. In works of type b)
efficiencies as high as those of central sewage works (see
2.2.2.2, Table 1) can be achieved, as long as they are properly
designed and operated.

By adding an underground
seepage plant, a sand filter pit or a soakaway,
the wastewater in works of type a) can undergo further biological
treatment and thus - provided the hydrogeological conditions are
suitable -passed to the subsoil. The discharge of wastewater
from works of type a) directly into surface waters is
generally not acceptable.

2.2.2.4 Impact on bodies
of water

Inadequately treated
wastewater may disturb the natural self-purification
capacity of receiving bodies of water based on
physical, chemical and biological processes and cause other
nuisances.

Undissolved
wastewater components cause deposits of sludge,
particularly in slow-flowing and standing waters, e.g. ponds,
lakes and shipping canals, and also in blind river arms, creeks,
reservoirs etc. If these originate from organic sediments, there
will also be decomposition phenomena with the development
of decomposition and fermentation gases, consumption of
the oxygen dissolved in the water by absorbed
decomposition products, inhibition of life forms or even mortality
of microorganisms and fish. Many organically polluted
commercial or industrial wastewaters favour the development
of "sewage fungus" in flowing waters containing
oxygen, particularly in cold seasons. Torn-off fungal particles
develop into fungal drifts and, where the current is weak,
frequently cause secondary sludge deposits with the
results mentioned above.

Dissolved and organic
constituents of wastewater require the presence of a certain quantity
of oxygen in the water for their biochemical breakdown,
where this occurs aerobically. This is determined in the same way
as in the wastewaters themselves and is hence called biochemical
oxygen demand.

Where the oxygen
content or the oxygen uptake capacity of a receiving body
of water is not sufficient for the biochemical oxidation of
the organic substances fed into it, their further breakdown
proceeds anaerobically. This results in the bacterial
reduction of nitrates, sulphates, oxygen-containing organic
compounds etc. to form carbon dioxide, hydrogen sulphide or
sulphides, ammonia, nitrogen and other decomposition products. Methane
is also formed in digested sludge (14). Anaerobic
decomposition processes caused by oxygen deficits disrupt
the largely aerobically structured self-purification capacity
of a body of water, and in serious cases may even cause it to
fail completely. Such problems can also occur in a body of
water under certain conditions even if treated wastewater is
discharged in the proper way, i.e. in accordance with the
rules of the art. One must then however assume that the self-purification
capacity of the body of water is insufficient with the
given discharge conditions. In such cases more stringent
standards must then be applied for wastewater discharges, in
order to meet or restore the required water quality targets (see
3.3).

Besides the problems
caused by lack of oxygen in biochemical oxidation, eutrophication,
i.e. the accumulation of plant nutrients in the water,
particularly phosphorus and nitrogen, is an important factor.
This "over-fertilisation" causes a mass development
of plant material, in particular water plants (foliage
plants) as well as blue, green and filamentous algae. Controlled
(aerobic) decomposition of the dying matter is then no longer
possible and this results in the above-mentioned anaerobic
water-polluting decomposition processes.

The water may be
polluted by a number of other substances having a toxic effect
on aquatic fauna besides phosphorus and nitrogen. These
include heavy metals, volatile halogenated hydrocarbons (e.g.
trichloroethylene), non-volatile halogenated hydrocarbons (e.g.
chlorobenzols), dioxins, pesticides and polycyclical aromatic
hydrocarbons (e.g. fluoranthene).

As aquatic fauna differ
in their sensitivity to environmental burdens or pollution, they
can be used as indices of contamination (bio-indicators).
This is particularly true of burdens caused by lack of oxygen
following the decomposition of organic matter and of toxic
burdens. The saprobic system is based on this (7).

2.2.3 Impact of disposal
of faecal matter

Much of what has been
said in 2.2.1.1 ("adverse effects of decentralised
wastewater disposal on the environment") apply here too. The
following typical (adverse) effects of the disposal of faecal
matter (in aerated/unaerated latrines, cesspools and
collection pits) should be mentioned here:

- health hazards due
to the use and the emptying of latrines and collection pits
(risk of infection due to direct contact with the faecal
matter; insect and rat infestation etc.)
- contamination of the subsoil, and particularly of the
groundwater too, if the hydrogeological conditions are
unfavourable
- health hazards upon final disposal (discharge) of the
faecal matter, where not carried out properly
- aesthetic and odour nuisance.

Beneficial effects: See
also 2.2.1.1 ("beneficial effects of decentralised
wastewater disposal on the environment").

2.2.4 Impact of
wastewater discharge

As stated in 1.4, wastewater
discharge means the return of wastewater to the natural
water cycle. This takes place in both disposal systems
(decentralised and central).

Regarding the impact of
wastewater discharge in the case of decentralised wastewater
disposal, see 2.2.1.1 and 2.2.2.4.

Apart from likely noise
and odour emissions, the effects of wastewater discharge in the
case of central wastewater disposal manifest themselves
mainly in the pollutant burden imposed on the receiving body
of water, caused by the discharge from the central sewage
works. Rainwater outfalls in a combined system can also
have an impact on bodies of water. Furthermore, what is
said in 2.2.2.4 also applies to the impact of wastewater
discharge in the case of central wastewater disposal.

2.2.5 Impact of sludge
disposal

2.2.5.1 Sludge disposal
in central wastewater disposal

The sludge
produced in a central sewage works must be treated
in the course of disposal. The most important treatment stage
is stabilisation; this may be carried out anaerobically or
aerobically (7), (8). In the case of anaerobic treatment
(digestion) of the sewage sludge, sludge digestion gases are
produced which are largely odourless if the digestion process is
carried out properly, i.e. through alkaline fermentation or
methane fermentation (8). The main gases produced are carbon
dioxide, nitrogen and methane.

Bearing in mind that
recycling is required wherever possible, agricultural use of
suitably pre-treated municipal sewage sludge should, if
possible, be regarded as the correct disposal strategy for
this sludge.

This should not however
lead to accumulations of heavy metals in the soil, as
these can pose a threat to people and animals via the food
chain, particularly in the case of the highly toxic heavy
metals cadmium and mercury.

If one considers the impact
of sewage sludges in terms of their value as a source of
raw material for agriculture, one should note (15):

- Sewage sludges
are valuable primarily as phosphate and nitrogen fertilisers,
but also because of their calcium and magnesium content; on
the other hand, the potassium content is negligible. The organic
matter content of sewage sludges also has a certain
value. It is therefore only logical to use recycled sewage
sludges for agricultural purposes.
- Sewage sludge which contains an excess of toxic
components or which may have other adverse effects must
not be used. Negative effects such as

·damage to soil organisms and plants
(phytotoxicity),

·damage to the health of humans and animals
as a result of excessive absorption through the food chain (via
accumulations in plants);

·adverse consequences for public hygiene

may be caused by an
excess of potentially toxic elements.

- The plant
availability of all components is a crucial factor. With
agricultural recycling of sewage sludges the

·phosphate content available to plants,

·nitrogen content available to plants,

·pollutant content available to plants,

are of prime importance.
The last of these is determined by the content of seven
potentially toxic non-ferrous metals (cadmium, chromium, copper,
lead, mercury, nickel, zinc) in sewage sludges, as well as in the
soils on which the sludge is to be spread.

Concerning the impact
of sewage sludges used in agriculture, in conjunction with production
of waste/sewage sludge compost, see also (16), (17), (18),
(19).

2.2.5.2 Sludge disposal
in the case of decentralised wastewater disposal

The sludge
produced in decentralised disposal systems in small sewage
works is mostly treated anaerobically. Where such a sewage
works is operated correctly, no significant odour nuisance
or hygiene problems should occur (20), (21). What is said
in 2.2.5.1 applies here too, particularly as regards sludge
disposal.

As the sludge to be
removed from small sewage works is not always uniformly
or sufficiently stabilised or sterilised (particularly in
the case of mixtures of faecal matter and carrying water in
wastewater collection pits), it may be a good idea to have secondary
digestion carried out centrally, e.g. in open earth basins
or tanks. This applies particularly where agricultural use
is envisaged. Such a simple method of secondary sludge
treatment is acceptablewherever the extra work
involved and the sometimes unavoidable odour emissions
are a comparatively minor problem.

2.3 Avoidance and
safety measures

2.3.1 Wastewater
avoidance

Wastewater which has not
been produced does not need disposal! In other words, the use of
appropriate procedures and measures to reduce the volume
or avoid producing wastewater takes pressure off the
capacities of wastewater disposal systems.

In the domestic
sphere, wastewater can largely only be avoided through water-saving
by the general public, e.g. through the installation and use
of water-saving sanitary installations, etc. Such measures should
not however be to the detriment of health and the proper
collection and removal of wastewater. This also depends on individual
citizens having the necessary motivation and
understanding, which can be promoted by means of appropriate
and regular information campaigns by the authorities and
corporations responsible for wastewater disposal.

The positive effect
of introducing progressive consumption tariffs on the
water-saving behaviour of the general public should not be
underestimated.

In the commercial and
industrial sphere, depending on the sector from which the
wastewater originates, specific plans should be developed to
reduce the volume of wastewater. Such considerations usually
centre on the recycling (multiple use) of process water,
if necessary with the help of efficient treatment measures.
Strict separation of sub-cycles is often highly advisable
in this regard (14), (22).

2.3.2 Safety measures

2.3.2.1 Introductory
notes

In this section, the
term "safety measures" is used to denote all
those measures which serve to minimise and compensate for
the environmental impact and, where applicable, to make up
for disturbances of the natural order.

2.3.2.2 Safety measures
in wastewater collection and removal

In the design,
construction and operation, primarily of central sewerage
systems, but also of decentralised sewerage systems, the objectives
should be as follows:

a) safe collection
and removal of sewage and rainwater, not least in order to
protect against disease
b) maintenance or improvement of the quality of surface water
and groundwater
c) construction of permanently watertight sewers and repair
of leaking sewers, pressure pipes and drains
d) optimisation of drainage works.

The above objectives
can be achieved in particular by the following measures or
procedures:

a)

- appropriate and
adequate dimensioning of sewers and storage chambers to
cope with peak flows (avoidance of flooding of
properties, roads and land)
- suitable routing of sewers and arrangement of outfalls (in
combined systems);
- flow control installations
- use of materials which fully meet the technical and
hygiene standards.

b)

- reduction of
discharge volumes (overflow frequency, discharge total,
duration, load) at the outfalls of combined sewerage systems
- elimination of faulty connections in dual systems
(with rainwater and sewage channels)
- reduction of volume of wastewater (rainwater, sewage
and mixed water) e.g. by rainwater percolation, cooling and
industrial water circuits in commercial and industrial
establishments, reduction in water consumption (see 2.3.1)
- prevention of water inflows from ditches, springs,
streams and drainage pipes (to be carried away only in
exceptional cases and only in rainwater sewers in the dual
system, with allowance for possible flooding).

c)

- use of high-grade
components (particularly pipes) and sealing
materials/sealing elements which behave well under long-term
stress. This prevents, on the one hand, penetration of
groundwater and percolation water into the sewerage network
and, on the other hand, leakage of wastewater and its
constituents into the subsoil, and hence into the
groundwater.

d)

- provision of qualified
and well-motivated personnel for monitoring, maintenance
and servicing work
- provision of adequate resources (sufficient tariffs)
to cover the costs incurred (23).

2.3.2.3 Safety measures
in wastewater treatment

To avoid harmful
pollution of the environment and particularly of surface
waters, the following principles in particular should be
observed:

- It is vital to
determine as accurately as possible the composition and
quantity of the wastewater produced and flowing to the sewage
treatment works, particularly taking account of the
short-term variations in domestic sewage quantities (daily
maximum, daily minimum), quantities and constituents of
commercial and industrial sewage (pre-treatment installations
may be needed on the industrial sites in question) and the
rainwater discharge conditions in the drainage area (7), (8)
- Reasonable allowance must be made for
climatological conditions (level and distribution of
annual rainfall, hours of sunshine, mean annual, monthly and
daily temperatures etc.)
- The treatment capacity of the sewage treatment works
must be appropriate to the ecologically acceptable and
use-related load capacity of the receiving water system,
paying close attention to the existing and anticipated
preloading.
- All relevant technical and health regulations must be
complied with when using treated wastewater and sewage sludge
on agricultural land.

Where it is necessary to
apply the technologically simplest wastewater treatment
processes, even if the work requires more land and
manpower, and particularly in countries with a hot and very
sunny climate, aerobic wastewater oxidation ponds without
artificial aeration are suitable (with or without a preceding
anaerobic stage). They have proved to be a highly successful
method of treatment (7), (8), (24), (25), (26), (27), (28). The operational
and ecological advantages of these systems are:

- simple management;
low maintenance and upkeep cost of system components
- discharge is highly suitable for irrigation purposes
- disinfection is quite adequate if the holding times
required by the system are adhered to (total reduction of
bacterial burden 97-98 % and more)
- low odour emissions under proper operating conditions and
low volume of stabilised sludge.

Furthermore, with a view
to safeguarding resources, particularly in countries with
a hot climate, closer attention should be paid to processes
utilising the valuable substances which the wastewater contains (wastewater
utilisation). These include various digestion processes (biogas
extraction), processes for agricultural exploitation (fertiliser
production) after adequate desludging and fish pond processes
(nutrient utilisation) (10), (29), (30), (31), (32).

Adverse effects
occur mainly when the principles listed at the outset are not
observed. In the case of oxidation ponds it is also worth
mentioning that although these have a good buffer capacity
to cope with sudden large quantities of wastewater, persistent operating
problems are to be expected if wastewater with toxic
constituents is delivered to the ponds, which in particular
causes damage to the aerobic biosystem. Several weeks may
pass before this is fully restored and the plant recovers the
required treatment capacity, during which time the receiving body
of water may be subjected to undesirable levels of pollution.

The following
environmental protection measures can be taken to combat
other non-water-related emissions:

- against noise:
e.g. enclosure of motors and blowers.
- against emissions into the air: Covering of treatment
basins; enclosure of treatment facilities such as automatic
rakes, preaeration basins, etc. The waste air must be
filtered (e.g. use of compost filter).
- treatment of the sewage sludge produced; aerobic
stabilisation, anaerobic stabilisation (digestion), drying.
Waste air or waste gas quantities produced must be filtered
and heat-treated if necessary.

In addition, the sewage
works may have to be landscaped in order to soften its visual
impact.

2.3.2.4 Safety measures
in sludge disposal

The sewage sludges
produced during wastewater treatment in municipal main sewage
works and in small domestic sewage works should - after treatment
- be recycled in some suitable way. For example, they may
be used to fertilise farm land. (see also 2.2.5). The same
applies to the contents of cesspits, subject to adequate (secondary)
treatment (see 2.2.5.2).

The composition of
sewage sludges in terms of their content of heavy metals and
non-degradable, sometimes toxic organic constituents is
often a problem. This applies mainly to indirect
discharges. Operators of public (central) wastewater disposal
systems must then take particular care to ensure that commercial
and industrial customers connected to the system discharge wastewater
which is harmless both for the operation of the central
sewage works and for the use of the sewage sludge on agricultural
land (see also Section 3).

One should start from
the principle that the sewage sludge is as "good"
or as "bad" as the wastewater produced at
the source. It is also important to monitor the indirectly
discharged wastewater as carefully as the directly discharged
wastewater, with particular emphasis on commercial and
industrial producers.

It is vital for all
community sewerage works initially to identify all commercial
and industrial indirect dischargers, where necessary to
demand suitable pre-treatment installations for the sites
in question and thereafter, at least on a random basis, to monitor
the discharge of the relevant plant.

It is also often a good
idea to advise indirect dischargers on process-related
wastewater management and wastewater avoidance and reduction, so
as to avoid emission problems from the outset.